Hyper-V and VHD Performance – Dynamic vs. Fixed

My name is Tim Litton, I work as a Program Manager within the Microsoft Windows Server team, and my particular area of focus is performance optimization for Hyper-V.

With the recent release of Hyper-V, customers are starting to ask us how to configure Hyper-V to get the best performance. It’s generally recognized that there is overheard running a virtualized environment, but the question that really needs to be answered is how much?

The following graph shows the relative performances for a number of different scenarios (with Dynamic VHD being the baseline).

Fixed VHD always performs better than a Dynamic VHD in most scenarios by roughly 10% to 15% with the exception of 4k writes, where Fixed VHD performs significantly better.

We ran 16 virtual machines when performing these tests (see “How We Tested” below) with the goal of evaluating how well Hyper-V performed in the server consolidation scenario.Being able to consolidate a number of physical machines onto a single machine and have the virtual machines able to handle the load is a very important design goal for Hyper-V.

The exact result that a customer is going to see will depend on quite a few variables (e.g. how large and often the reads and writes are, how many outstanding I/O there can be at one time), so performing real-world testing is the best way to assess what impact virtualization will have.

Tim Litton posted a blog about performance optimization for Hyper-V, looking at fixed versus dynamic VHDs. Here’s an excerpt: The following graph shows the relative performances for a number of different scenarios (with Dynamic VHD being the baseline).

Would you see the same peformance degradation if you pre-grew the VHD (let’s say a 36 GB partition) with a filler file (let’s say 10 GB) and then deleted it. That way, you would get around the possible file fragmenetation (which is probably causing the
performance issue with the dynamic disks)?

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With Dynamic Disks there is an extra translation done to map the logical disk location within the Dynamic VHD to the physical backing file location so Fixed Disks will always be faster than Dynamic Disks.